Known in the art is an internal combustion engine providing, in an engine exhaust passage, an NOx storage catalyst storing NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich, in which internal combustion engine the cylinders are divided into a first cylinder group and second cylinder group and an exhaust passage of the first cylinder group and an exhaust passage of the second cylinder group are merged and connected to a common NOx storage catalyst (for example, see Japanese Patent Publication (A) No. 8-189388). In this internal combustion engine, usually, in all cylinders, combustion is performed with a lean air-fuel ratio. The NOx produced at this time is stored in the NOx storage catalyst. On the other hand, if the NOx storage capacity of the NOx storage catalyst approaches saturation, the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst is made temporarily rich so as to release NOx from the NOx storage catalyst and reduce it.
In this regard, fuel and lubrication oil contain sulfur. Therefore, the exhaust gas includes SOx. This SOx is stored together with the NOx in the NOx storage catalyst. However, this SOx is not released from the NOx storage catalyst just by making the air-fuel ratio of the exhaust gas rich, so the amount of SOx stored in the NOx storage catalyst gradually increases. As a result, the storable NOx amount ends up gradually decreasing. Therefore, when the SOx amount stored in the NOx storage catalyst increases, the NOx storage catalyst must be made to release the SOx.
In this case, the NOx storage catalyst can be made to release the SOx if making the temperature of the NOx storage catalyst rise to the substantially 600° C. or more SOx release temperature and making the air-fuel ratio of the exhaust gas flowing into the NOx storage catalyst rich. However, in the above-mentioned internal combustion engine, if making the air-fuel ratio of each cylinder of the first cylinder group rich and making the air-fuel ratio of each cylinder of the second cylinder group lean, the large amount of unburned HC exhausted from the first cylinder group is oxidized by the excess oxygen exhausted from the second cylinder group in the NOx storage catalyst. The heat of oxidation reaction at this time may be used to raise the temperature of the NOx storage catalyst. At this time, if maintaining the amounts of unburned HC and the amounts of excess oxygen exhausted from the cylinder groups at the optimum amounts in accordance with the operating state of the engine, that is, if maintaining the rich degree of the first cylinder group and the lean degree of the second cylinder group at the optimum degrees in accordance with operating state of the engine, the temperature of the NOx storage catalyst can be maintained at the SOx release temperature.
Therefore, in the above-mentioned internal combustion engine, the rich degree of the first cylinder group and the lean degree of the second cylinder group required for maintaining the temperature of the NOx storage catalyst at the SOx release temperature are stored for each operating state of the engine. When SOx should be released from the NOx storage catalyst, the rich degree of the first cylinder group is made the stored rich degree in accordance with the operating state of the engine and the lean degree of the second cylinder group is made the stored lean degree in accordance with operating state of the engine.
However, when trying to make the rich degree of the first cylinder group and the lean degree of the second cylinder group the stored rich degree and lean degree corresponding to the operating state of the engine, the injection timing, ignition timing, etc. must be matched, so massive matching operations become necessary for each rich degree and lean degree set for each operating state of the engine. Therefore, as an actual problem, with this method, there is the problem that it is difficult to hold the temperature of the NOx storage catalyst at the SOx release temperature.